I'll link to some resources, because there is no quick and easy way to find the right agent/publisher for you and your project, but this is where you should begin:
What have you written? Non-fiction or fiction? Mainstream, literary, genre, cross-over? Kinda-sorta like these other books that have been published, or totally other?You may feel -- and you may feel correctly -- that what you've written is art or at least high craft, but you're now stepping from the realm of creation into the world of business. You've passed from making into selling, and you need to do all you can to sell that book.
The first step is to analyze where it would fit in a bookstore. Where would it be shelved? How big is the market liable to be, assuming the publisher does nothing to promote it, which may very well happen. Sometimes publishers just throw a book at the wall and see if it sticks. It may be up to you to identify your market and get the word out about your book.
Do you want to approach an agent or a publisher? Do you want a small press or a major publisher?Major publishers used to have slush piles: stacks of unsolicited submissions that were farmed out to by-the-piece readers or given to junior associates for an initial glance, to see if a book was worth bothering with. Thanks to cuts in staffing and expenditures, major publishers often rely on agents to be their first readers, and don't accept unsolicited submissions. (See the next point.)
Small presses sometimes take unsolicited submissions, but small presses are usually small, meaning they don't have a large editorial staff (Per Bastet has only me, God help us), so they may also discourage unsolicited submissions. They may accept submissions by invitation only or by pitch sessions at conventions. This isn't out of snobbishness, but because they have to limit the number of titles they publish each year and don't want to be overwhelmed. Small presses are also usually run by writers, who know how it feels to be put on hold for months and don't want to subject other writers to that limbo. (See the next point.)
Some agents take clients who don't already have a track record, some don't. Some take unsolicited submissions, some only take submissions from people recommended by current clients, publishers, or other professionals. Conventions/workshops sometimes hold pitch sessions for agents as well as for publishers. (See the next point.)
Now comes the next point:
RTFG -- READ THE GUIDELINESWhen you decide where you want to submit, look them up online (most, if not all, agents/publishers have an online presence these days). Compare what you may have found in print to what you find online. Folks switch places of business, people get promoted, and what was up-to-date in Writer's Market 2016, printed in 2015, might or might not be the case in January 2017.
Whether the guidelines say so explicitly or not -- and they shouldn't have to say so -- do not send a rough draft. Do not send a first or second draft. Send a finished, polished, spell-checked, grammar-checked, read and reread work that you would be delighted to pay cash money for.
BE ABSOLUTELY CERTAIN that the place you're sending your work represents or publishes that sort of work. Format your query, proposal, sample chapters, or manuscript the way the place you're submitting wants it submitted. If you don't follow those first guidelines to present your work the way they want it presented, they won't trust you to take critiques and suggestions for rewrites well.
Expect critiques and suggestions for rewritesThis isn't about finding an agent or publisher, but bear it in mind. Your agent/editor will have opinions or house guidelines they'll want your book to conform to. Your first reaction will probably be, "Oh, HELL, no! This is my book! This is my book! This is my book! This is my book!" After you do that, consider the suggested changes.
If you absolutely cannot make a particular one (or ones), ask if you may make a case for letting it stand. If the agent/editor says, "My way or the highway," you may need to cut bait and take your work elsewhere. You have to balance artistic integrity against getting a reputation for being difficult to work with.
~*~That's plenty to digest for right now. Here are some resources:
How to Publish a Book is a WikiHow article (wif pitchurs), that leads you, step-by-step, through the processes from manuscript to marketing.
Finding a Publisher vs Finding an Agent is a good article on Novel Writing Help, a website with a world of information and helpful instructions.
Although SFWA stands for Science Fiction & Fantasy Writers of America, good advice is good advice, and their How To Find a (Real) Literary Agent is only one of many pieces comprising their Writer Beware series.
Over and over, you'll see writing advice sites recommending QueryTracker. There's a reason for that: They have good stuff there. Finding a Reputable Agent or Publisher is well worth your time.
Last, I recommend your reading Lois Winston's post on Marilyn Meridith's blog on her happy journey from a "real" publishing house to self-publishing.
Marian Allen, Author Lady
Fantasies, mysteries, comedies, recipes